This question throws out the idea of how aliens, perhaps a passing-by alien who comes to investigate a potentially habitable planet from orbit, would recognize an 'official' communication from Earth. It deals with the political and societal concerns of ensuring messages sent out are from a legitimate source, but bypasses a much more pressing concern.

How would aliens close enough to recieve remote signals from Earth actually recognize communication as anything but white noise? How would they be able to tell from within our Solar System (inside the Ort cloud, let's say) that our random and undirected signals were in fact signals from intelligent life, without knowing what frequencies or broadcasts those signals are actually supposed to represent?

What if we knew aliens were passing by, and wanted to ensure the signal was better recognized? How could we guarantee that these aliens would recognize our signals as coming from an intelligent life form, and not just random cosmic noise?

3 Answers
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First off, I want to cut down one of your words: "How could we guarantee that these aliens would recognize our signals..." we cannot guarantee anything. Current prevailing linguistics opinions indicate that it is impossible to guarantee another mind recognizes anything at all. The best we can do is give our best effort.

One thing we have looked towards is how we can make a signal look as unlike any known physical sources as possible. Clearly outputting spectra that line up with our sun's emissions is not going to get a message across. We look for things we can do that are unusual.

One of these things we can do is send a very narrow band signal. Most physical sources generate very broadband spectra because they are not actively trying to tune it (with an exception of spectral lines, which end up being tuned to the atoms themselves). There is a standing theory that one of the best ways to send this signal is a narrow band signal focused around a particular frequency at which interstellar hydrogen is unusually transparent. We cannot think of any reason why a natural process would select this frequency, so any signal on that frequency is a solid sign that something unusual put it there.

Another key trick is to keep the bandwidth of the message down and use "simple" modulation schemes. We can transmit data much more efficiently using things like CDMA and QAM, but those modulation schemes look like noise unless you know the decoder ring (in fact, their noise-like appearance is something they designed towards to minimize interference issues). Simple AM modulation is the most likely bet, or perhaps simple FM modulation.

The lower the bandwidth of the message, the more our signal can look like a simple carrier wave at first glance (and thus is less likely to be lost in the noise when alien SETI is looking at many gigahertz of bandwidth). We should keep the message simple. So far, we believe a mathematical message is the simplest. A simple encoding of prime numbers is a popular way to send that message. We know of no natural processes which cause prime numbers to appear, so a signal which counts 2, 3, 5, 7, 11, 13... would look tremendously unusual to anyone listening. Other sequences like Fibonacci have been suggested as well.

High quality repetition is also helpful. There are not many things in nature which send reliable messages with low jitter. We look at pulsars and quasars because they are unusual exceptions to the rule, so there's a good chance the other species is looking for them too. If we can demonstrate a message which is repeated to a tremendous precision (1ppm or something similar), it means the more they look at the message, the more unusual its timing will appear.

There has been discussion of harmonics as well. There has been interest in possible sending the "root" of a chord on the hydrogen line, along with a third and a fifth. This one has been debated (it isn't clear if thirds and fifths are a human thing, or a natural law), but it is out there.

$\begingroup$How would you send a mathematical sequence on the airwaves (e.g. with AM)? Would we have to assume that the alien interceptors know and implement some decoding scheme?$\endgroup$
– horse hairMar 29 '15 at 14:07

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$\begingroup$@horse hair: That is an EXCELLENT question, which could be studied dozens of ways in a book, so I didn't want to pin any author down into one of them. The movie Contact explored this question. There, large blocks of time between sinewave bursts was used, similar to the spaces between dots and dashes in morse code. This approach is reasonable because the sine waves and the spaces appear to be universal to physics.$\endgroup$
– Cort AmmonMar 29 '15 at 16:25

$\begingroup$I think it could also be used as a checksum. Obviously sending just a string of numbers only goes so far. Eventually we want to send more complicated data in a more complicated encoding. After attracting their attention with the series of primes in morse code, you might encode a signal in a more complicated way, and make sure part of the data encoded is also the primes. That way, when the aliens do stumble upon the right decoding mechanism, another unusual string of primes shows up, perfectly correlated with the first one. It's not a guarantee, but its a start.$\endgroup$
– Cort AmmonMar 29 '15 at 16:27

$\begingroup$Cool, thanks. At what distance from earth does our most powerful transmission for this purpose fall below the noise floor? After that point our announcing ourselves to aliens signal becomes pointless, right?$\endgroup$
– horse hairMar 29 '15 at 16:56

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$\begingroup$That is another good question, and it depends on a tremendous number of variables (its worth looking into Shannon's Law if you're curious). Generally speaking, one of the preferred techniques is to transmit on a band that hydrogen does not absorb at. It travels far that way, and we don't see very many natural processes that emit on that frequency, so the noise floor is lower in that band.$\endgroup$
– Cort AmmonMar 30 '15 at 3:12

This concept is basically that in any particular communication, entropy is the probability that you can guess what's coming next based on what you've already received. So in English, if I start a sentence with, "Hi, how" - then there's a high probability that the next word will be "are". Structured communications of any kind have a relatively low entropy. Alternatively, white noise/signals are entirely random, and are thus impossible to predict. So they have a high entropy.

The neat thing about Entropy is that it's a property of information, not of a particular language. You do need to sample the signal for a while (so you have a baseline) but you don't need to actually understand anything - you can use Shannon Entropy analysis to detect if any sort of communication has meaning, given enough baseline, and to distinguish it from white noise.

Note: A goal of encryption is to make it impossible to detect patterns, so encrypted communications have high entropy and will be difficult to distinguish from white noise. Presumably our communications with entirely new aliens won't be encrypted, but it might make it harder to detect random signals that are picked up by accident, assuming the senders are following basic security practices.

$\begingroup$Even encrypted communication is however normally focused. Take WiFi (802.11 b/g/n/x/y/z/etc) as an example. That is often (but not always) encrypted, and there may be multiple layers of encryption. However, if you scan the upper end of the UHF range (which is 0.3-3 GHz, whereas WiFi is often found around 2.4 GHz) with a sufficiently directional antenna you will see significant signal strength peaks about 20 MHz wide coming from specific locations. I believe the only form of communication which would be truly difficult to distinguish from white noise would be encrypted UWB transmissions.$\endgroup$
– a CVn♦Mar 5 '15 at 9:45

$\begingroup$Frequency-hopping transmissions such as those sometimes used by military radio equipment is another type of transmission which would be difficult to pick out, but if you have a sufficiently good receiver and are scanning across the relevant frequency range, you will again be seeing signal strength peaks showing up here and there across time and frequency. You might not recognize at all what those mean, but then again that's how we discovered pulsars. (Note that this all supports your argument, it does not detract from it.)$\endgroup$
– a CVn♦Mar 5 '15 at 9:50

Think of it this way: You're in the forest, there are lots of sounds. After spending a lot of time there you have a very good idea what natural sounds exist in the forest. Now you hear a chainsaw. Woah, you think, that's not normal. I'm going to check that out.

It's the same with the universe. There are lots of signals floating around out there, we detect them all the time, we become accustomed to what causes them. Now we detect a signal that has structure, like an amplitude modulated radio wave. Woah, we think, that's not normal. Let's check that out.

Space faring aliens would do the same thing. They know what the background space 'sounds' like and when something sticks out, they'd likely investigate.